As a platform through which a telecommunication carrier can flexibly and securely provide an IP multimedia service to a user, an IMS (IP Multimedia Subsystem) has been standardized (Non-Patent Document 1).
For example, a mobile network operator (which is referred to as an “operator,” hereinafter) constructs an IP-based network (a core network) by the IMS to provide a mobile communication service to a user through an access network, such as those of an LTE/3G. Additionally, by establishing an IP interconnection (which is referred to as an “interconnection,” hereinafter) between operators, communication between users subscribing to different operators is enabled.